


Dungeon Diving

by Zemmiphobia



Category: Harry Potter - J. K. Rowling, ワンパンマン | One-Punch Man
Genre: Alternate Universe - Hogwarts, Golem!Genos, M/M, Magic, Pre-Relationship, Super-Powered-Magical!Saitama, Teen!Saitama, obligatory
Language: English
Status: Completed
Published: 2015-11-08
Updated: 2016-01-04
Packaged: 2018-04-30 14:54:46
Rating: General Audiences
Warnings: No Archive Warnings Apply
Chapters: 2
Words: 3,429
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/5168018
Author URL: https://archiveofourown.org/users/Zemmiphobia/pseuds/Zemmiphobia
Summary: <blockquote class="userstuff">
              <p>Saitama is a bored seventh year in search of adventure when he finds a little more than he expected hiding in the dungeons below the school. What happens next is the start of a strange and, most likely, odd friendship.</p>
            </blockquote>





	1. Chapter 1

**Author's Note:**

> Because every fandom needs a Harry Potter crossover, right? I live to serve. This is Saigenos, just at the start of the relationship. I'll probably add more in the future. Probably.

Saitama strolled through the corridor with a negligence that would have bordered on obscene if it hadn’t been for the alert spell drifting around him like a cloud. It wasn’t that he was worried about being caught. Most of the teachers paid as much attention to the average looking seventh year as they did to the house elves. It was more a healthy desire to find whatever, hopefully powerful, monsters lurked in the lower dungeons before they found him. It wouldn’t do to kill them before a fight could properly start, after all.

Not many people knew about this part of the dungeons, a second floor that had been boarded up years before. Saitama had found it completely by accident after he had taken a wrong turn on his way to the Slytherin dorms. He had had to go into an empty classroom, slip and fall through a rotten door behind a tapestry, and trip over a trap door before finding the staircase that led down to the lowest level of the castle.

So here he was, first day of Christmas break, carrying a bag of supplies down a dusty hallway with only a _lumos_ to guide him. Many people would say it was a stupid idea to go alone but despite being friendly with most of the school, there was no one Saitama would call a friend and certainly no one he would trust with a secret like this. There were some adventures that just weren’t meant to be shared. Besides, the sign at the door had made it very clear that this level had been blocked off for a reason. Bringing another student with him would likely end with the poor soul getting picked off before they had even reached the end of the hallway.

Lost in his thoughts, Saitama almost missed the soft scratching noise coming from somewhere beyond him. He forced a little extra magic into his _lumos_ spell but the source remained hidden. Smiling with anticipation, he edge forward, eyes peeled for whatever was waiting for him. He turned the corner and just had enough warning from his alert spell to duck the large furry body that lunged out of the darkness.

Rolling, he slid to the side and landed on his feet, arms raised and legs braced. The creature, massive in the dim light, shook itself and turned to face him, scuttling like a spider.

At first, Saitama’s brain couldn’t put the features together. The creature had no head, only a massive body, but as he stared at it, he realized a face was watching him hungrily from the fur; two cold, slitted eyes, and a gaping mouth from which a large wet tongue darted out, tasting the air. Once he noticed the face, the five legs, instead of the eight he had expected, were easy to spot.

“Quintaped.” He breathed with excitement. Merlin only knew how it got down here, in a school full of children, hundreds of miles away from its home island, but all that mattered was that it had, and it wanted to fight him. He watched it shift its weight from side to side, tensing its thick muscles to pounce. Even with the weak light, the teeth gleamed evilly, each nearly as long as his forefinger.

Grinning a little wildly, he raised a hand.

“ _Locomotor Wibbly_ _!_ ” he snapped, flinging out the weak red light.

The Quintaped exploded in a shower of gory red bits with clumps of orange hair clinging to them.

Saitama stared.

“Shit.”

Sighing, he waited for the pieces to stop, falling before moving forward and bringing his _lumos_ spell closer. It was no use, the monster was dead.

Annoyance ripped through his veins as he slapped a hand over his eyes and silently consigned whatever power was laughing at him to hell.

When that failed to help, he swore, spun on his heel, and punched the wall.

Or, what was actually a moldy door covered in enough dust to make it look like a wall. Which promptly splintered.

Without the recoil from his fist meeting solid stone, Saitama’s body followed the movement, leaving him sprawled awkwardly in the doorway of what appeared to be an abandoned laboratory.

The room was large, twice the size of Professor King’s potions classroom. It was clearly not a room for students, however, since it was crammed with tables, bookshelves, and a large glass container full of green liquid. Everything was liberally covered in dust but Saitama could still make out the shapes of potions equipment, Arithmancy tools, and strangely, several large muggle machines that looked like they belonged in one of the superhero comics his cousin had given him for Christmas.

A quiet _Incendio_ set the gutted candles on the wall alight and only set one book on fire. Extinguishing it quickly, Saitama began to explore the room, his monster quest temporarily forgotten.

The books on the shelves were neat, despite the dust, and had obviously been well cared for. No sign of mice or pests, which was not surprising given what had been guarding the door.

“But why guard an empty room.” He asked aloud, voice distorting oddly in the total silence. “A few books and some machines?” The machines in question were broken, laying in several pieces, and old. He wasn’t exactly up to date on modern muggle science but he was pretty sure, given how long this place had been closed off and how quickly muggles advanced, they probably weren’t worth much anymore.

He turned and stared up at the large class container in the center of the room. The liquid inside was green, murky, and looked as if it has spent some time inside a troll’s stomach. It was making him a bit queasy just looking at it, honestly. But something was grabbing his attention, drawing his eye back to the glass each time he looked way. It took him a moment but it finally dawned on him.

Protective runes had been etched into the glass, circling it all the way around. Runes to prevent the glass from breaking, runes to keep the inside temperature consistent, even a rune to create bubbles, of all things. He traced them with a finger, smiling at the attention and care that had been put into them. Whoever had worked here had obviously cared a great deal about this thing.

A sudden dip make him pause, revealing a rune he didn’t recognize. He pulled his _lumos_ spell closer, until it was almost touching the glass. It was strange, a twisting bastardization of the rune for human, gold, and life, all twining around each other. Reaching into his bag, he pulled out a muggle notebook and quickly jotted the new symbol down.

He’d be a poor Slytherin if he didn’t recognize an opportunity when he found one.

So intent on his discovery, he didn’t noticing the pricking sensation of being watched until he looked up and saw the face staring at him from inside the glass.

Squeaking like a first year in surprise, he dropped the notebook and skittered backwards. The face did not blink, but it did tilt to the side, much like a curious dog.

Saitama forced his heartrate down and studied the face. It was… well, it was _almost_ human. It was the face of a young man, blond hair swaying in the murky liquid like seaweed, with two black scleras and gold irises that glowed. As he watched, two hands appeared, pushing up against the glass.

Reaching out, Saitama tapped the glass.

The man tapped back.

Inappropriate amusement rose in the back of Saitama’s mind and he tapped out a beat. The man tapped the beat back and then reached down and tapped at the large metal pieces that were attached to the outside of the container.

“…You want me to open it?” Saitama asked after a moment. The man nodded.

On the one hand, it was a terrible idea to free what was probably a monster that had been locked away here with very good reason. On the other hand, Saitama couldn’t find it in him to care. Squatting down, he studied the metal, trying to find the latch. Maybe his day was finally looking up.

~

Or maybe not.

Even after nearly half an hour of searching, prying, pushing, and hitting buttons, the metal refused to budge. Saitama had moved from amusement, to annoyance, to full blown frustration.

“Would you just fucking open?!” He yelled at the contraption as the man stared down at him in silence. Groaning, he stood up and stretched out his sore back. “Ugh, who made this thing? A fucking sphinx?” The man shrugged and stared sadly at him. “Hey, don’t tell me you don’t know how to open it either?”

The man shook his head and stared to push away from the glass, disappearing back into the murky liquid.

“Hey! Where are you going!? I didn’t spend thirty bloody minutes staring at pipes so you could go back to napping!”

The man shook his head again, pointed at the runes, and then shrugged. Saitama slapped his forehead.

“Shit, why didn’t I think of that sooner?” The man tilted his head but before he could gesture at the teen, Saitama stepped back, put up a shield, and shouted, “ _Confringo,”_

The man’s eyes widened as he shoved himself backwards, just as the spell connected with the glass, sending shards and liquid flying. Even then, the man flew backwards and thudded heavily against the back wall.

“Shit!” Saitama said again, running around the broken container. “Should’ve tried to tone it down a bit. You still breathing?”

The man, though now Saitama could see he was entirely made of metal and some thick, muscle like substance, opened his eyes.

“Thank you. I did not believe I would ever be free.”

“I’ll take that as a ‘yes’, then?” Saitama crouched down and helped pull him up into a sitting position. “You got a name?”

“Genos. I am the golem created by Professors Kuseno to hunt dark wizards.” Genos gripped Saitama’s hand between his own, and stared at the teen solemnly. “I was an Auror during the Third Wizard War but I was heavily damaged towards the end, after my hometown was attacked by Dark wizards. I was forced to stay in St. Mungo’s for many years before the Professor tracked me down and offered me a part in his techimagi experiments. I agreed, willing to do anything to track down the dark wizards who had escaped prosecution and avenge my—

“Whoa, whoa, whoa. Man, calm down. I asked if you had a name, not a tragic backstory. Sum it up in twenty words or less, or I’m gonna leave you down here.”

“My apologies, Master.”

Saitama froze half way through standing.

“Uh, what now?”

Genos tilted his head. Despite the lack of expression, the golem radiated confusion.

“Please clarify, Master.”

Saitama took an undignified step back, hands up. Genos followed him, lurching awkwardly to his feet.

“That. What the hell? I’m not your master.”

“Professor Kuseno has not returned to his laboratory in some time. Judging by the amount of dust, I believe he has either been killed or been forced to leave. In his absence, I need a master. Golems are unable to function without a magical source to feed and guide them.”

“So you need a babysitter?”

“I would not have phrased it as such but the sentiment is not inaccurate.”

“And you’re just going to follow me around, feeding off my magic like a parasite?” Genos flinched and bowed his head, looking the picture of abject misery.

“My deepest apologies. I did not realize the discomfort this would cause you. My scans estimated your power level to be much higher than Professor Kuseno’s, I did not believe it would cause you harm.”

Saitama sighed.

“Don’t worry about it. I guess I’ve got enough to spare.” He turned and motioned Genos to follow him. “Come on, I’m getting kinda sick of all this gloom. Let’s head up to the kitchens. I’ll bet you’re dying for a real meal.”

“Golems do not need to eat.” Genos informed him, before adding, “However, I have always enjoyed udon.”

“I’m sure the house elves can make you some, they’re used to a varied menu.” They headed for the stairs, Genos trailing behind Saitama like a large child.

“Man, how am I going to explain this to Professor Aoi?”


	2. Chapter 2

**Summary for the Chapter:**

> Saitama has friends... maybe. She's not sure yet.

“That’s adorable,” said a voice above him, “but I don’t think you’re allowed to smuggle in your muggle boyfriend.”

Saitama opened one eye and glared blearily at his housemate, who was grinning at him in a frankly disturbing way. He wished he could just roll over and go back to sleep and would have if the object of her amusement wasn’t curled up against his back, one heavy arm thrown over his chest, and sound asleep.

“It’s really cute though, how devoted you must be to get him past the wards.”

“Piss off.” He muttered, pushing the limb off him and sitting up. The golem, still in the process of recharging, did not react. Saitama rubbed his face. “Should I even ask why you’re in the boy’s dorm?”

“Oh, I’m sorry, am I’m intruding in your man cave?”

“Just don’t make Chadha cry again. It’s too fucking early.”

“It’s two in the afternoon.”

“Too. Fucking. Early.” Saitama repeated carefully as he slid all the way out of his bed. He grabbed a muggle shirt from the floor and pulled it over his head, ignoring Fubuki’s wrinkled nose. Slipping his wand into his pocket, he strolled past her and into the common room.

“Hey, wait! Where are you going? Are you seriously just going to leave him in there?”

“He’ll live.”

“He’s a _muggle_ and well I’m perfectly content to leave him be, I’m sure Sneck is going to have Professor Mamoru in here. You know how much he dislikes you.”

“I’m pretty sure Mamoru doesn’t remember I exist.”

Fubuki rolled her eyes, following him as he climbed a short flight of stairs.

“ _Sneck_ , idiot. He’s been thinking of ways to get you expelled since _first year_.”

Saitama paused in the doorway, tapping his chin.

“Sneck. Sneck. Is that the kid with the embarrassing snake themed pants?”

“How should I know, I’ve never seen his knickers— What are you doing!?” She shrieked as Saitama pulled the covers off a bed and slipped into its mint scented sheets.

“Going back to bed.” He said as he lay down his head and closed his eyes. “Wake me up when its dinner.”

“Get out of my bed this instant!” Fubuki screeched, darting forward to rip the covers off only to slam into a barrier and get tossed back on her ass. Knowing Saitama well enough to guess that he had put up a silencing charm as well as his ridiculously overpowered ward, she stormed off to find one of her underlings to rant at.

~

Genos came to awareness with two thoughts in his mind. One, that his master’s bed was empty and two, he was being watched. Stiffening, his eyes snapped open and he thrust out an arm towards the unknown presence. His attack was awarded with an undignified squawk of alarm and the thud of a body hitting the floor.

“For fucks sake,” said a cultured female voice from below him. “I’m starting to see what they say about couples.”

Genos blinked and leaned over the bed where a young woman was glaring at him through her short, messy hair.

“Going to steal my bed too?”

“My apologies, I attacked without thinking. Have I injured you?” The woman huffed and got to her feet, brushing nonexistent dust from her robes.

“Please, as if a little tap like that would hurt me. I’m in the top ten of the dueling club, I’ll have you know.” She put her hands on her hips and stared at him until he started to feel slightly uncomfortable. Just as he was about to ask for Saitama’s whereabouts, she spoke.

“I’ve never met a muggle before. You’re better looking than I expected.”

Genos straightened, bowing his head apologetically.

“Forgive me but I am not a muggle, I am a golem.”

“A golem?” She sat down beside him, eyes wide. “I thought you were all wiped out in the war.”

She let out a squeak as he whipped his head around and clamped his hands over her shoulders, pulling her closer.

“There are more of me?”

“S-sure, I mean, maybe? There were a lot of golems made during the third war but no one’s seen any for, uh, at least two decades.”

Genos released her to stare dramatically down at his hands in silence.

After a moment, she coughed and patted him lightly on the shoulder.

“Well, uh, I suppose that explains why I couldn’t feel any magic from you.”

“You are correct,” he said softly, not looking up, “golems cannot produce magic.”

More silence.

“Uh, so, did Saitama create you?”

“No. I was created by Dr. Kuseno.”

She blinked at him, opened her mouth, and then closed it. After a moment, she stood and held out a hand.

“Well come on. I’m getting you back to Saitama. There is no way this is going to be my mess.”

“You know where my Master has gone?” He asked over her sudden snort of laughter.

“Yeah, he’s stolen my bed, the prick.”

She pushed him along in front of her, waving cheerfully at a fellow Slytherin who stopped to stare at them in surprise.

“What does he intend to do with it?” Genos asked in confusion, allowing the teen to steer him up the steps and into the girls’ dorm, only pausing when the privacy wards briefly tried to push him back out.

The girl laughed as the light of the magic washed over him and died.

“Saitama really knows how to find them.” She said, shaking her head.

Just as he opened his mouth to ask her to clarify, his master’s shining head caught his attention. Instantly the girl was forgotten as he lept towards the bed.

~

Fubuki laughed as the golem hit the barrier and bounced back, laughed harder when the rebound caused him to fly into the trunk of one of her more bitchy housemates and stopped laughing all together when the pressure on his ward caused Saitama to jump awake and break the canopy of her bed.

“You better fix that!” She demanded as Saitama stood on her bed, blinking at the sudden fabric wrapped around his shoulders. He smirked at her.

“Mah, Fubuki, don’t tell me you can’t even do a little cleaning spell.”

“I know more cleaning spells than _you_ , bastard. Clean it up or I’m going straight to Tatsumaki about your new boyfriend.”

She would never, of course. Even if she didn’t avoid her older sister like the plague, Slytherins stuck together. Especially when one of them discovered priceless magical antiques.

The annoying overpowered teen rolled his eyes at her but obligingly jumped off her bed and cast a weak cleaning spell at it, which managed to not only fix the canopy but straighten her side table and polish her trunk as well. On the floor, the golem rolled onto his knees and bowed to her housemate, the picture of an ancient warrior before his sensei.

Well, she supposed, at least one of them looked the part. Saitama, as usual, looked like an embarrassment to everyone but himself.

“You really need to stop crawling into bed with me, ya know?” The teen was saying as he scratched his stomach. “It’s not really made for two.”

“My deepest apologies, Master. I believed you looked cold and wished to provide you with warmth. It has been many years since I was human, but I was given important information about the human body to aid me in my battles and –

“Would you please stop calling me that?!”

“I have no other way to address you, my programming dictates that I maintain a professional relationship with my master—

“As adorable as your domestic is,” Fubuki cut in drily, “are you going to introduce me?”

The two boys started, having forgotten she was in the room. Which, she thought, was rather amusing considering it was the _girls’ dorm_.

“Uh yeah. Fubuki, this is Genos, Genos this is Fubuki.”

“I am pleased to be formally introduced to my master’s friends.” Genos said with a bow in her direction.

“ _Friends_!” She snorted, sending a powerful stinging hex into a laughing Saitama, which caused him to laugh harder. “He wishes!”

END

**Notes for the Chapter:**

> The wards let in the boys in because neither of them are attracted to females. In my headcanon, the wards are able to tell the intent of individuals are keeps them out based on their reason for entering. If the person has no sexual interest or intent to harm, they will be allowed in. Genos is briefly stopped because of Saitama, who is inside.

**Author's Note:**

> Well, there you go! Saitama is younger, of course, only seventeen. So I feel that he's bored but not quite at the level of apathy that a 25 year old Saitama could achieve. With Genos, however, Saitama finally has an exciting life waiting for him after graduation.


End file.
